1. Technical Field
The present invention relates to an apparatus for and a method of transferring signals between two devices. The invention has particular application in portable, battery-powered devices such as personal digital assistants, mobile telephones, tablet PCs, web browsers, etc.
2. Related Art
Inductive digitiser systems used in portable computer devices work by transferring power to a moveable stylus inductively. When powered, the stylus in turn transmits a signal back to the digitiser tablet and this is detected by a number of detecting elements to determine the position and status of the stylus. The return signal power needs to be sufficiently high relative to noise sources to yield position and status information acceptably free from noise, such as position jitter. Additionally, it may also be desirable to power electronic circuitry in the stylus, which requires additional power. The digitiser tablet must therefore emit sufficient magnetic field to provide this power to the stylus. Where the power source is a battery, it is desirable to emit this magnetic field efficiently, using as small an amount of power from the battery as possible, within the constraints of cost and space.
There are a number of existing digitiser systems that inductively power a stylus. U.S. Pat. No. 4,878,553 discloses a system that powers the stylus using an array of loop coils or windings which are arrayed over an x and y direction. The system powers the stylus by passing current through those loop coils in closest proximity to the stylus. However, this arrangement is highly inefficient because there are a large number of loop coils, each of which is formed from a small number of conductor loops printed on a printed circuit board (PCB) and therefore the width of copper available for each loop coil can only be small. Additionally, the printed circuit board process itself limits conductor thickness to typically 0.05 mm, so that the overall volume of copper per loop is relatively small. As is well-known to those skilled in the art, such a small volume of copper results in the system being relatively inefficient at powering the stylus when it is above the coil.
WO 00/33244 describes another digitiser system for use in a personal digital assistant or a mobile telephone, where the stylus is powered by a single printed winding that is wound around the perimeter of the printed circuit board. In this system, a larger proportion of the printed circuit board area is devoted to this excitation winding, yielding greater copper mass and hence efficiency. However, the printed circuit board is usually installed behind a display with a metal bezel and a layer of magnetic screening material is provided to shield the digitiser signals from processing electronics beneath the printed circuit board. With this arrangement, eddy current losses in both the screening material and the bezel represent significant sources of power loss. Although the bezel may be slit to prevent such eddy current losses, this is undesirable due to a weakened display housing, the greater potential for electrostatic damage to the display and the need for customised display mechanics.
Another problem experienced by this type of stylus digitiser system is the tilting of the stylus by the user during use, which results in a position offset in the position measurement. Most current systems try to overcome this problem by using dedicated algorithms which process the signals from the loop coils or sensor coils to determine the tilt and hence the position offset error. However, these tilt correction algorithms rely on signals from conductors which are placed on either side of the actual position of the stylus and work well in the centre of the measurement area. However, when the stylus is positioned over an edge of the measurement area, the tilt correction algorithms become less accurate (because windings are only available to one side of the stylus) resulting in less accurate position measurements at the edge of the measurement area.